Vacuum boosters are commonly used to provide power braking assist for vehicle operators in brake apply. Such boosters use a differential pressure actuator to boost the driver exerted force on a brake pedal or similar brake input device. The differential pressure actuator works from a source of low pressure fluid, such as a vacuum line from the engine intake manifold and a source of relatively higher pressure: the atmosphere. These two fluid pressure sources are controlled in a standard double valve arrangement involving a power piston connected to the differential pressure actuator output member and carrying a low pressure valve seat, an air valve responsive to brake input and carrying a working pressure valve seat, and a floating valve member engagable with either of the valve seats and always in contact with at least one.
It has further been disclosed to provide in alternate activation capability in such a booster with electrical activation by a linear electric solenoid acting within the booster. Such alternate activation provides the capability for automatic power brake activation independent of vehicle operator input. However, the linear solenoid designs tend to be large, heavy and expensive to manufacture, due to the generally low forces generated by linear solenoids. Not only must the solenoid itself be of significant size, but the seal or seals between members, one of which is moved by the solenoid with respect to the other, must be of a low friction design. In addition, a linear solenoid is an inherently non-linear device with a short effective range. The magnetic force exerted by the coil on the armature is strongest when they are together and falls off rapidly as they are separate. This significantly limits the effective range of the arrangement. Also, a linear solenoid is inherently a two position device. Although pulse width modulation of the activating current can produce an approximately continuous output, it is only an approximation. More precise control in electrical activation would be desirable.